1. Field of the Invention
The present invention relates generally to a wavelength converting apparatus and an optical cross connect system adapting thereof, and more particularly to a wavelength converting apparatus and an optical cross connect system adapting thereof, in which a probe beam used for wavelength conversion is generated by optical sources having fixed wavelengths, thus allowing its costs to be low and wavelength conversion to be facilitated.
2. Description of the Prior Art
As a high speed data exchange service or wideband image service has recently been commercialized, there is great demand for large transmission capacity for a communication network. In addition, large-scale transmission is required for the construction of a super high speed communication network that is pursued to improve national competitiveness. In order to cope with the above-described demands, many methods of increasing communication speed have been suggested up to now. These methods are generally classified into three types: Time Division Multiplexing (TDM) to increase the communication speed by increasing electric circuit speed, Optical Time Division Multiplexing (OTDM) by making short pulses optically and multiplexing the pulses, and Wavelength Division Multiplexing (WDM) by employing various wavelengths and transmitting them through an optical fiber.
In TDM communication technology, apparatuses of several Gbps speed have been already developed, but it is known that communication speed faster than several Gbps is very difficult to achieve. In OTDM technology, technology of extracting clocks from high-speed multiplexed pulses is very complicated, so putting the technology to practical use faces a lot of problems. In WDM transmission technology, communication employs many wavelengths over a wide range of wavelength, so nowadays this technology is studied most actively.
In comparison with conventional communication technology in which only one wavelength is carried and transmitted on an optical fiber core, the WDM transmission method maximally utilizes a wide wavelength range (that is, a transmissible range) in such a way that signals are carried on channels arranged at a uniform wavelength interval, the various channels are optically multiplexed and transmitted through an optical fiber, and the channels are separated from one another and separately used by wavelength difference at the receiving side of the optical fiber.
In the WDM transmission method, an operation of multiplexing optical signals of various wavelengths into an optical fiber is called “Multiplexing,” while an operation of separating a plurality of optical signals from one optical fiber is called “Demultiplexing.” In a current WDM transmission method, wavelengths of about 1310 nm and 1550 nm are mostly employed.
When a transmission network is set up using an optical transmission system to which the WDM transmission method is applied, adding and dropping of wavelengths should be performed at the nodes of the transmission network. The adding and dropping are generally carried out by an optical cross connect system.
FIG. 1 shows such an optical cross connect system. As shown in this drawing, the optical cross connect system employs an optical switch 5 between N demultiplexers 3 and multiplexers 7. A signal amplified by an optical preamplifier 1 is demultiplexed into respective signals by the demultiplexer 3, and the paths of the demultiplexed signals are selected by the optical switch 5. The signals from the optical switch 5 are multiplexed by the multiplexer 7, and the multiplexed signals are amplified by an optical amplifier 9 and connected to an output optical fiber.
In the WDM, a plurality of channels are arranged in a wide bandwidth and a plurality of signals are transmitted through the channels. Accordingly, in order to transmit signals, each signal having wavelength suitable for standard transmission, or to prevent interference between channels, the wavelengths of each channel must be precisely controlled. Furthermore, signals to be transmitted to any output link must be transmitted while each signal has a different wavelengths.
To this end, in the optical cross connect system, a wavelength converting apparatus is required to convert the wavelengths of signals from the demultiplexer 3 or the wavelengths of the signals into the multiplexer 7 to appropriate wavelengths. The wavelength conversion apparatus converts the signal wavelength to the wavelength of the probe beam through cross gain modulation, cross phase modulation, or optical-to-electrical-to-optical (OEO) conversion. The wavelength converting apparatus can be positioned after the demultiplexer 3 or in front of the multiplexer 7 of the optical cross connect system as shown in FIG. 1.
FIG. 2 shows a conventional wavelength converting apparatus that is used in conjunction with the conventional optical cross connect system. In this drawing, a conventional wavelength converting apparatus is shown, which is used for the conventional optical cross connect system having N links. As shown in this drawing, in the conventional wavelength converting apparatus, optical signals S propagate into M Wavelength Converters WCs provided in each link, and probe beams, each probe beam having a specific wavelength, also propagate from tunable optical sources at the same time. In FIG. 2, a laser diode is used as an example of such an optical source. Each of the optical signals S and each of the probe beams are injected to each of the WCs for cross gain modulation, cross phase modulation, or OEO conversion, so a signal having a desired wavelength can be produced from each of the WC according to the wavelength of the probe beam applied.
As described above, through the wavelength converting apparatus, the wavelength of the optical signal is converted according to the wavelength of the probe beam applied. In order to convert the signal wavelength, the tunable optical source changes its wavelength which is applied to the wavelength converter as a probe beam. Generally, the output wavelength of the tunable optical source is changed by controlling current and temperature. That is, to get a wavelength desired from the tunable source, it is required to compare the output wavelength with the intended wavelength and to adjust the current and the temperature.
In general, a probe beam produced from a tunable optical source is affected by surrounding temperature or current. Even though the wavelength of the probe beam from the tunable optical source is converted by a change of current or temperature, the converted wavelength may not be the wavelength desired because the change in temperature and current can make the converted wavelength drift away from the wavelength desired. Therefore, to keep the wavelength within a certain tolerable range of wavelength, regardless of temperature change in environment and undesired current change in the circuit, a locking apparatus is required.
This WC, however, requires a complicated control process. That is, to convert a wavelength, the WC disenables the locking function, converts the wavelength, and then locks the wavelength, enabling the locking function at the desired wavelength. This whole wavelength conversion process requires a complicated control algorithm, taking a relatively long time for the wavelength conversion.
In addition, as shown in FIG. 2, if the optical cross connect system consists of N links, each link having M channels, N×M tunable optical sources and wavelength locking controllers are required in order to convert wavelengths corresponding to each of the channels, requiring not only complicated control but high cost.